Not Applicable
The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of a contract awarded by an agency of the U.S. Government.
Not Applicable
1. Field of the Invention
The invention pertains to thin film acoustic devices. More particularly this invention pertains to thin film, bulk wave acoustic resonators for use as filters at microwave frequencies. A thin film, bulk wave acoustic resonator typically utilizes a thin layer of piezoelectric material that is sandwiched between two thin conducting layers of material to form a resonator. The conducting layers serve as electrodes and when an electrical voltage, at a microwave frequency, is applied between the two electrodes, the consequent electric field between the electrodes interacts with the piezoelectric material to generate acoustic waves within the piezoelectric material. In a bulk wave, acoustic resonator, acoustic waves propagate in the direction normal to the thin layers of material and the electrical impedance between the two electrodes exhibits a resonance when the acoustic thickness of the combination of the piezoelectric layer and of the two electrodes is approximately one-half of an acoustic wavelength or an odd multiple thereof. In some instances the acoustic waves are acoustic shear waves and in other instances the acoustic waves are acoustic longitudinal waves.
2. Description of the Prior Art
The fabrication of piezoelectric resonators for use at microwave frequencies is well known in the prior art. See, e.g., the descriptions of such devices in the specification of U.S. Pat. No. 5,894,647 for a xe2x80x9cMethod for Fabricating Piezoelectric Resonators and Productxe2x80x9d, Lakin, and see the references to prior art cited therein. See also xe2x80x9cMicrowave Acoustic Resonators and Filters,xe2x80x9d by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139; Guttwein, Ballato and Lubaszek, U.S. Pat. No. 3,694,677; and xe2x80x9cAcoustic Bulk Wave Composite Resonatorsxe2x80x9d, Applied Physics Letters 38(3) by Lakin and Wang, Feb. 1, 1981. Such resonators also may be fabricated on, and supported by a substrate by including a set of intervening layers of material having alternating high and low acoustic impedances, which layers have thicknesses of a quarter wavelength. The intervening layers act as an acoustic mirror that acoustically isolates the resonator from the underlying substrate. See, e.g., U.S. Pat. Nos. 3,414,832 and 5,373,268 and 5,821,833 and 6,291,931. For methods of analysis and further descriptions of reflectors and resonators see Lakin, xe2x80x9cSolidly Mounted Resonators and Filters, 1995 IEEE Proc. Ultrasonics Symposium, pp. 905-908 and Lakin et al. xe2x80x9cDevelopment of Miniature Filters for Wireless Applicationsxe2x80x9d, IEEE Trans. on microwave Theory and Techniques, Vol. 43, No. 12, December 1996, pp. 2933-2939.
As depicted in FIG. 1 hereof, U.S. Pat. No. 5,821,833, for A Stacked Crystal Filter Device and Method of Making, Lakin, disclosed a bulk acoustic wave, stacked crystal filter 100, a supporting substrate 112 and an acoustic reflector 113 located between stacked crystal filter 100 and the substrate. Acoustic reflector 113 was made of a sequence of layers 108, 109, 110 and 111 of material having alternately high and low acoustic impedance. The stacked crystal filter comprised two layers 102 and 106 of piezoelectric material separated by a conducting electrode layer 103 and bounded on the top and bottom by conducting electrode layers 104 and 107. The top and middle electrodes provided a signal input port 101 and the middle and bottom electrodes provided a signal output port 121 with the middle electrode layer 103 being connected to signal ground 105. The stacked crystal filter exhibited high transmission of signals from the input port to the output port for the signal frequency at which the combined thicknesses of the two piezoelectric layers and of the three electrode layers constituted approximately one-half an acoustic wavelength. The stacked crystal filter, by itself, also would have transmitted frequencies which were approximately an integral multiple of said frequency for which the combined thicknesses were approximately an integral multiple of one-half an acoustic wavelength.
The thickness of each layer of material in the reflector was one-quarter of an acoustic wavelength at the frequency at which the stacked acoustic resonator had a thickness of one acoustic wavelength and at this frequency the upper surface of the reflector exhibited a very low impedance that reflected substantially all of the acoustic wave from the resonator incident upon the reflector. As a consequence the reflector facilitated the transmission of signals by the stacked acoustic resonator at that frequency. However, the transmission by the filter of signals at higher frequencies for which the resonator thickness was two, three or more times a half acoustic wavelength, were inhibited because at these higher frequencies the layers of material underlying the stacked crystal filter did not operate as a reflector and did not isolate the acoustic vibrations of the stacked crystal filter from the underlying substrate.
A different example of prior art is depicted in FIG. 2. In this example a pair of stacked crystal filters 200 and 201 were mounted side by side upon a reflector comprising layers 208, 209, 210 and 211 mounted on substrate 212 and connected electrically together in the manner depicted in FIG. 2 to provide a filter in which the input port 205 to stacked filter 200 and the output port 221 from stacked filter 201 are both located on the upper surface of the device. Electrode 204 and the underlying portions of piezoelectric layers 202 and 206 and the underlying portions of electrodes 203 and 207 comprise the two resonators within stacked filter 200. Electrode 224 and the underlying portions of piezoelectric layers 202 and 206 and the underlying portions of electrodes 203 and 207 comprise the two resonators within stacked filter 201. As depicted in FIG. 2, electrodes 203 and 207 each constitute parts of both stacked crystal filters and provide direct electrical connections between the two stacked crystal filters. Because there are no intervening layers between the two resonators within each stacked crystal filter (other than the conducting electrode 203, the degree of the coupling between the two resonators in each stack is fixed and may not be adjusted. As a consequence the range of filter characteristics that may be achieved by this configuration is limited.
U.S. Pat. No. 3,568,108 for a xe2x80x9cThin Film Piezoelectric Filterxe2x80x9d, Poirier, disclosed the use of piezoelectric semiconductors for use in resonators. The patent places special importance upon the fact that the resonator in the patented device utilizes piezoelectric layers which are also semiconductors. The patent specification states that it is a characteristic of piezoelectric semiconductor materials that an acoustic wave propagating through the material generates a piezoelectric field which interacts and exchanges energy with mobile charge carriers driven through the medium by an external DC electric field and states that when a DC voltage is applied to the medium it creates a direct current [col. 1, ln. 48-57]. The sole independent claim of the patent recites resonators that include an epitaxial film having both piezoelectric and semiconductive properties [col. 4, ln. 25-26]. Accordingly, the ""108 patent discloses resonators that utilize piezoelectric that are also semiconductors having semiconductive properties. The ""108 patent does not disclose resonators that use piezoelectric materials that are insulators.
The ""108 patent discloses a filter comprising an input resonator and an output resonator and the specification states that the input rf electrical signal is filtered by virtue of the different acoustical frequency-amplitude characteristics of the (two) resonators [col. 4, ln 5-9]. Unfortunately the specification does not clearly identify the elements that comprise each resonator and the nature of these elements. At one point the specification appears to disclose two resonators that are in surface contact with each other [col. 3, ln. 31-33 and see the areas encompassed by numbers 41 and 42 in FIG], while, in apparent contradiction, independent claim 1 recites means disposed between the resonators [col.4, ln. 35-36] and dependent claim 4 describes said means as comprising a plurality of layers [col. 4, ln. 52-54]. At one point the specification indicates that each of the layers of material that intervene between the resonator electrodes has a thickness of between (2n+1)/4 and (n+1)/2 acoustic wavelengths [col. 3, ln. 52-54], while at another point the specification indicates that each of these intervening layers has a thickness of one-half acoustic wavelength [col. 3, ln. 63-65]. As a consequence, it is difficult to determine with certainty just what art was disclosed in the specification.
In any case the specification states that the extend (sic-extent) to which the frequency-amplitude characteristics of the resonators overlap substantially determines the electrical characteristics of the filter [col. 4, ln. 7-9]. It is not apparent whether the foregoing statement is based on the fact, or perhaps the assumption, that an interaction between the rf electric fields and a DC flow of mobile carriers within the piezoelectric semiconductors would cause the coupling between the resonators to be unidirectional, thus causing the resonators to exhibit a filter characteristic that was simply the product of the resonance properties,of the two resonators, or instead was based upon an assumption that the two resonators were only weakly coupled to each other. Or perhaps this statement was based upon some other undisclosed element or characteristic of the device. In any case, the patent does not disclose the use of an insulating piezoelectric material in the resonators, and does not disclose the use of intervening layers of material between a pair of resonators, the parameters of which intervening layers are selected to control the coupling between the resonators so as to produce a filter whose transfer characteristic is not simply the product of the frequency responses of the individual resonators.
U.S. Pat. No. 4,349,796 for Devices Incorporating Phonon Filters, Chin et al., disclosed the use of a superlattice of one-hundred alternating layers of GaAs and AlGaAs. The superlattice was located between a superconducting tunnel junction that generated acoustic wave phonons at one end of the superlattice and a second superconducting tunnel junction that was located at the other end of the superlattice and detected phonons that passed through the superlattice. The patent specification described a superlattice, in which each layer had a thickness of one-quarter of an acoustic wavelength and that selectively reflected phonons to produce a quasi-monochromatic source of phonons. The patent also described a superlattice, in which each layer had a thickness of one-half of an acoustic wavelength, that selectively transmitted phonons through the lattice to provide a quasi-monochromatic source of phonons. The present invention differs from the device disclosed in the ""796 patent because the present invention uses acoustic resonators, instead of superconducting tunnel junctions, as input and output devices. Furthermore, the present invention uses a small number of intervening layers of material to adjust the bilateral acoustic coupling between the acoustic resonators so that the frequency response characteristics of the closely coupled resonators produce the desired filtering properties between the input and output ports of the device. In contrast to the present invention, in the ""796 device the superlattice, itself, provides the frequency selective properties of the device.
The present invention utilizes thin-film, bulk acoustic wave resonators that use piezoelectric materials that are insulators and not semiconductors and that, as a consequence, avoid the debilitating losses that otherwise would result from the use of semiconducting piezoelectric materials. The terms xe2x80x9cinsulatorxe2x80x9d or xe2x80x9cinsulatorsxe2x80x9d are used herein with respect to a piezoelectric material as meaning a material for which the conductivity is low enough such that the interaction of any conductive current with acoustic waves propagating at microwave frequencies within the material would be insufficient to create a significant asymmetry in the acoustic properties of the material and would normally not constitute the major loss mechanism for acoustic waves propagating through the material. For the purposes of this invention, an insulator means a material that has a dielectric relaxation frequency that is less than one-tenth of the frequency of the acoustic wave propagating through the material.
In this device, the layers of material intervening between the two resonators are selected to control the acoustic coupling between the two resonators so that the coupled resonators produce a filter transfer characteristic, S21, that is more complex than simply the product of the frequency responses of the two, individual resonators. By adjusting the degree of the coupling between the resonators to be substantially equal to or greater than critical coupling, the transfer characteristic for the filter can be specially adapted to many applications. For example, the transfer characteristic of this device can have a broader peak and steeper sides than could be obtained by using two resonators that are only weakly coupled together or for which the coupling between the resonators is not bilateral.